This invention relates to a polarity judging arrangement for use in a reception section of a digital transmission network comprising a transmission section and a transmission line between the reception and the transmission sections.
The digital transmission network is typically an ISDN (integrated services digital network). The transmission line is typically a two-wire metallic cable. The transmission section transmits a time division multiplexed digital signal to the transmission line. The time division multiplexed digital signal may be a multilevel code signal.
The time division multiplexed digital signal comprises successive multiframes each of which comprises first, second, . . . , and N-th frames, where N represents an integer greater than two. The first frames of each multiframe comprises a multiframe synchronization signal representative of a first predetermined synchronization word and a transmission data signal which succeeds the multiframe synchronization signal. The first predetermined synchronization word consists of at least one symbol.
Each of the second through the N-th frames of each multiframe comprises a frame synchronization signal representative of a second predetermined synchronization word and another transmission data signal which succeeds the frame synchronization signal. In the ISDN, the second predetermined synchronization word is generally defined by inverting the first predetermined synchronization word.
The reception section receives the time division multiplexed digital signal from the transmission line as a received digital signal. The received digital signal has a true polarity equivalent to a polarity of the time division multiplexed digital signal when the transmission line of a two-wire metallic cable is correctly or normally connected in polarity between the transmission and the reception sections. When the transmission line is incorrectly or inversely connected in polarity between the transmission and the reception sections, the received digital signal has an inverted polarity relative to the true polarity.
In order to correctly decode the transmission data signal in the reception section no matter whether the transmission line is connected correctly or incorrectly in polarity between the transmission and the reception sections, a polarity judging arrangement is required which is for use in the reception section and for judging whether the received digital signal has the true polarity or the inverted polarity.